Rotary kilns are long, slightly inclined cylinders used for processing materials such as lime, limestone, dolomite, magnesite, petroleum coke and cement. The material to be treated is introduced at the higher end and heated air flowing counter-current to the material is introduced at the lower end. Rotary kilns generally operate on a twenty-four hour basis for several months between scheduled down periods.
Rotary kilns typically have a refractory brick interior and a steel shell exterior, and some have at least one heat exchanger. The heat exchanger divides the cross section of the kiln into three or more segments to enhance the heat transfer from the gas to the material and improve mixing of the material. A three-segment heat exchanger comprises three spokes or legs which extend from the axial center of the kiln to locations equally spaced around the interior circumference of the steel shell. Commercially available three-segment heat exchangers have been sold under the trademark Trefoil®.
Rotary kiln heat exchangers encounter harsh operating conditions. For example, internal gas temperatures may typically be 1,000 to 3,000° F. in a highly basic atmosphere in a rotary lime kiln, although temperatures outside of this range are possible depending on the particular application. The heat exchanger must take the structural loading and erosion, e.g., from several hundred tons per day of partially calcined rock that slides across or falls against the surfaces of the heat exchanger. Furthermore, the heat exchanger rotates continuously with the kiln, which subjects the components of the heat exchanger to varying compressive and tensile forces. The heat exchanger must also withstand the kiln shell deflection upon revolution over its roller supports.
Conventional rotary kiln heat exchangers are typically from 8 to 16 feet long along the longitudinal kiln axis, depending on the kiln diameter and other parameters, and have spokes or legs typically from 9 to 13.5 inches thick. The heat exchangers are usually formed from individual refractory bricks, although some have been formed in-situ from refractory materials which are cast and cured inside the kiln. Installation of conventional brick heat exchangers is labor-intensive and requires specially skilled artisans. The bricks also require complicated forms specific to a single rotary kiln size to support them during construction. Thus, brick heat exchangers are slow to install and are expensive. In-situ cast refractory heat exchangers also suffer from disadvantages such as premature wear, complicated forms and slower installation than brick.
Some examples of rotary kiln heat exchanger designs are disclosed in U.S. Pat. No. 3,030,091 to Wicken et al., U.S. Pat. No. 3,036,822 to Andersen, U.S. Pat. No. 3,169,016 to Wicken et al., U.S. Pat. No. 3,175,815 to Wicken et al., U.S. Pat. No. 4,846,677 to Crivelli et al, U.S. Pat. No. 5,330,351 to Ransom et al. and U.S. Pat. No. 6,257,878 to Marr et al.
Despite these prior designs, a need still exists for a rotary kiln heat exchanger that is relatively fast and simple to install, and can withstand the harsh operating conditions of rotary kilns for extended periods of time. The present invention has been developed in view of the foregoing, and to address other deficiencies of the prior art.